A full duplex two-wire communication link comprises two full duplex modems connected by a single two-wire cable, such as for example a twisted-pair, over which each modem simultaneously transmits data signals to and receives data signals from the other modem. Generally, the data signals that are transmitted by a modem are samples, determined using an appropriate sampling function and sampling rate, of data symbols in a stream of data symbols that the modem generates which represent the actual data being transmitted by the modem. The samples are generated by sampling a product of the sampling function and the symbols at the sampling rate as they stream through a sampling window of suitable width in symbol periods (i.e. at any one time the sampling window contains an integral number of symbols). The data symbols are symbols from an appropriate symbol constellation and are organized in the data symbol stream in symbol frames.
The transmitter and receiver of each modem in the communication link are generally connected to the twisted-pair via a circuit known in the art as a hybrid circuit. In optimal operation, the hybrid circuit minimizes energy from signals transmitted by the modem's transmitter from reaching the modem's receiver and prevents energy from received signals from reaching the modem transmitter. The output impedance of the modem's hybrid also terminates the twisted-pair. Optimally, the hybrid of each of the modems terminates the twisted-pair with impedance substantially equal to the impedance of the twisted-pair.
In general some energy from a signal transmitted by a modem “leaks” through its hybrid circuit to the modem's receiver and generates an echo of the transmitted signal at the modem's receiver. In addition, the impedance of a twisted-pair can vary in time as a result of changes in temperature and humidity, changes in stray capacitance of the cable or physical damage to the cable. The output impedance of a hybrid can also vary with changes in temperature and humidity. Optimal termination of a twisted-pair by a hybrid is therefore generally difficult to achieve and maintain. As a result, when one modem in the two-wire link transmits a signal to the other modem, some of the energy in the transmitted signal is reflected back to the receiver of the transmitting modem. The reflected energy, as does the energy that leaks through the hybrid, generates echoes of the transmitted pulse at the transmitting modem's receiver.
Incoming data signals received by the modem are therefore usually mixed with and corrupted by echoes from a plurality of outgoing data signals that the modem itself has transmitted at times prior to the reception of the incoming data signal. In general, the amount of reflected energy in echoes that the modem receives is substantial and energy received from near echoes can often be greater than the energy of incoming data signals.
Most modems in a full duplex two-wire communication link therefore generally process incoming data signals to reduce the amount of energy from echoes that corrupts the incoming data signals. Various “echo canceling” methods for reducing echo contamination of data signals received by a modem are known in the art. Generally, these methods involve generating a simulated echo that provides an estimate as a function of time of the echoes that the modem receives from data signals that the modem transmits. When a data signal is received by the modem, the simulated echo is subtracted from the received data signal to correct it for corruption by echoes.
The simulated echo at a particular time is generally assumed to be a linear function of a plurality of data signals transmitted by the modem at times prior to the particular time. The function is defined by a set of coefficients, each of which multiplies the amplitude of one of the plurality of transmitted data signals to determine the contribution of the transmitted data signal to the simulated echo at the particular time. The coefficients, hereinafter referred to as “echo cancellation coefficients”, are determined for each modem of the communication link during a half duplex training period in which the modem transmits data signals and the other modem is silent. During the training period signals received at the transmitting modem comprise only echoes of its own transmitted data signals and noise. The echo cancellation coefficients are determined so that after subtraction of the simulated echo from incoming signals during the half duplex training session, the output of the modem receiver is substantially due only to channel noise (which cannot be canceled by the simulated echo). The determination of the echo cancellation coefficients for a modem during the half duplex training period and generation of the simulated echo during subsequent full duplex communication are generally performed by an appropriate processor comprised in the modem. The processor receives signals proportional to data signals transmitted by the modem and signals from the output of the modem receiver and processes the signals it receives to determine a suitable set of echo cancellation coefficients. Usually, the echo cancellation coefficients are determined using a least squares minimizing algorithm that determines values for the echo cancellation coefficients that minimize the square of the receiver output signals during the half duplex training period.
Following determination of the simulated echo cancellation coefficients for each modem, the two-wire communications link is operable to provide full duplex communication between the two modems with echo cancellation correction of data signals transmitted between the modems. As long as the echo characteristics of the communication link are stable and do not change substantially, use of the echo cancellation coefficients that are determined in the training period provides relatively efficient echo cancellation. If however, echo characteristics of the communication link change substantially, echo cancellation using the coefficients can be degraded and quality of communication, as measured for example by eye quality monitoring (EQM) and post detection signal to noise ratio (PDSNR) reduced.
If quality of communication becomes unacceptable due to changes in echo characteristics of the communication link, full duplex communication over the link is stopped and a new training period is initiated to update the echo cancellation coefficients for the modems and renegotiate data transmission rates. Each new training period generally interrupts communication between the modems for a relatively extended period of time. Such interruptions are not acceptable for communication applications that require uninterrupted data flow between the modems.
A method for updating cancellation coefficients without interrupting full duplex communication over a two wire communications link is described in “A New Digital Echo Canceler for Two-Wire Full-Duplex Data Transmission” by K. H. Mueller, IEEE Transactions on Communications; Vol. Com-24, No. 9, September 1976, the disclosure of which is incorporated herein by reference. The method is also described in “The Theory and Practice of Modem Design” by John A. C. Bingham; copyright 1988 by John Wiley and Sons Inc.; pp 371-373, the disclosure of which is incorporated herein by reference. Whereas the method described in the above references does not require stopping full duplex communication, the method is relatively slow. As a result, it is not efficient for updating echo cancellation coefficients in situations for which echo characteristics of a communication link change relatively rapidly.